Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrotreating is a process in which hydrogen is contacted with a hydrocarbon stream in the presence of hydrotreating catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen and metals, such as iron, nickel, and vanadium from the hydrocarbon feedstock.
Residue or resid streams are produced from the bottom of a fractionation column. Resid hydrotreating is a hydrotreating process to remove metals, sulfur and nitrogen from an atmospheric residue (AR) or a vacuum residue (VR) feed, so that it can be cracked to valuable fuel products.
Hydrotreating of resid streams requires high severity. Resid desulfurization units typically have hydrodemetallization (HDM) catalyst up front, followed by hydrodesulfurization (HDS) catalyst. Slurry hydrocracking (SHC) cracks resid streams to valuable streams. Hydrogen is necessary for these reactions to proceed. Recovery of excess hydrogen from the reaction zone typically occurs from a separator overhead line which carries a vapor stream highly concentrated in methane. The separation of hydrogen from methane complicates hydrogen recovery.
In residue hydroprocessing units, hydrogen is not easily fully recovered from the cold separator vapor stream. Too little heavier hydrocarbon liquid is present in the cold separator to absorb sufficient methane from the vapor stream to achieve the required hydrogen partial pressure if recycled to the hydroprocessing reactor. Consequently, a portion of the cold vapor stream has to be purged from the process to avoid excess methane accumulation, thereby sacrificing valuable hydrogen. Several designs have been attempted to remove methane from this cold vapor stream, but none have proved economical. Recycling cold flash drum liquid to the cold separator has not economically absorbed methane because large volumes of cold flash drum liquid have to be pumped and recycled to the cold separator to absorb sufficient methane from the cold separator liquid. Adding the cold vapor purge stream as additional gas feed to a steam methane reformer hydrogen plant has not been economical because most of the cold vapor stream is hydrogen which adds to the volume of material that must be heated, cooled and passed through the reformer but it is already the product that is to be produced rendering inefficiencies. Pressure swing adsorption (PSA) has not proved economical in separating methane from hydrogen gas in the cold vapor stream because the PSA vent gas pressure is too low requiring another stage of compression to recycle the hydrogen to the hydroprocessing unit which adds to capital and operational expense. Moreover, PSA is touted for high selectivity, but lower recovery of hydrogen; whereas high hydrogen recovery is necessary to make recovery of hydrogen from the cold vapor purge stream economical. Consequently, refiners typically route the cold vapor purge stream and all of its hydrogen to the fuel gas header to be burned as low grade fuel.
Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success in hydrogen removal from methane. The membranes most commonly used in commercial gas and liquid separation applications are asymmetric polymeric membranes having a thin nonporous selective skin layer that performs the separation. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. Membrane separation is believed to have higher recovery of hydrogen from methane, but lower hydrogen selectivity.
It would be highly desirable to efficiently recovery hydrogen from a resid hydroprocessing process.